The present invention relates to a method of bonding semiconductor elements and a junction structure.
Higher performance can be achieved for semiconductor devices by bonding individual semiconductor elements. Representatively, for solar cells that are photoelectric conversion semiconductor elements, a broad sunlight spectrum can be absorbed by layering solar cells having a different band gap to form multiple junctions. This can improve photoelectric conversion efficiency.
In general, these multi-junction solar cells have a monolithic stack structure in which a III-V group semiconductor cell (GaAs based) is formed on a GaAs substrate or a Ge substrate by bulk growth. In this case, a performance in which power generation efficiency is 40% or more can be achieved by using a Ge or InGaAs system sensitive to a long wavelength range as a bottom cell. However, since the combination of these materials is a lattice mismatch system, their growth procedure is complicated, contributing to a cost-increasing factor.
Meanwhile, a smart stack structure, which has attracted much attention in recent years, has a structure in which two or more cells are mechanically bonded in various fashions to allow various cells to be easily combined, representing a key technology for the next-generation solar cell in view of high performance and lower cost. In the smart stack structure, it is important to obtain a junction structure having not only electric conductivity but also transparency assured at a bonding interface for each solar cell. Further, obtaining optical characteristics favorable for solar cells is also important equally to or not less than assuring transparency.
Conventionally, as a method of bonding semiconductor elements including solar cells, an approach is known in which an electrically conductive adhesive, i.e., an organic polymer resin comprising a micron-sized particulate metal compound and a metal nanowire is used for bonding as described in, for example, Patent Documents 1 and 2.
Further, for example, as described in Patent Document 3, a method of bonding semiconductor elements has been reported in which using electrically conductive nano particles covered with organic molecules having a diameter of 100 nm or less, low temperature sintering between electrically conductive nano particles is performed taking advantage of the melting point depression based on nano-sizing.
However, by the above approach, a junction structure having electrical conductivity and transparency assured at a semiconductor element interface is difficult to obtain for the following reasons. Moreover, optical characteristics favorable for solar cells are also difficult to obtain.
First, in the case of Patent Documents 1 and 2, electric conductivity may be decreased, or activity may be lost because contact failures of particulate metallic compounds and metal nanowires may occur due to the thermal expansion of an organic polymer resin induced by heat generated from the element itself at the time of instrument operation after bonding, changes in ambient air temperature and the like. Moreover, in order to maintain light transmission, the concentrations of the particulate metallic compounds and the metal nanowires need to be kept low, which is unfavorable for electric conductivity.
Next, in the case of Patent Document 3, the electrically conductive nano particles having a diameter of 100 nm or less, which are used herein, are usually covered with a protective layer comprising organic molecules in order to improve handling properties. However, in order to obtain good electric conductivity after bonding, something has to be done to prevent these organic molecules remaining after sintering.
Moreover, as described above, in order to maintain transparency or light transmission at an interface, the concentration of electrically conductive nano particles also needs to be low and uniformly present at the interface to prevent the generation of a large sintered compact of the electrically conductive nano particles. However, bonding itself can be difficult because a decreased particle concentration leads to a decreased sintering frequency.
Meanwhile, although not a technology for bonding semiconductor elements such as solar cells each other as described above, applications for used in elements have been considered in which metal nano particles are arrayed in two dimensions on a substrate surface using an amphiphilic block copolymer as a template in expectation of the quantum size effect (see Patent Documents 4, 5). However, no exploration has been attempted for electroconductively bonding semiconductor elements each other through metal (electrically conductive) nano particles arrayed on a surface without using an adhesive such as organic molecules and an adhesive material.
Further, although not a technology for bonding semiconductor elements such as solar cells each other as described above, applications have been considered in which a nano structure is created by transferring a thin film of a metal and the like deposited on another substrate surface by the vapor deposition method and the like using a stamp having any 3-dimensional pattern (Non-Patent Document 1) for use in a sensor element and the like (Non-Patent Document 2). However, even in the case of the stamp technology, no exploration has been attempted for electroconductively bonding semiconductor elements each other through metal (electrically conductive) nano particles arrayed on a surface without using an adhesive such as organic molecules, and an adhesive material.    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-309352    Patent Document 2: Japanese Patent Application Laid-Open No. 2011-138711    Patent Document 3: Japanese Patent Application Laid-Open No. 2004-107728    Patent Document 4: Japanese Patent Application Laid-Open No. 2006-88310    Patent Document 5: WO2007/122998    Non-Patent Document 1: Loo et al., Journal of the American Chemical Society, 124 (2002), 7654.    Non-Patent Document 2: Hatab et al., ACS Nano, 2 (2008), 377.